Wireless networks have become a focus of attention, as systems free from wiring used for known wired-communication methods. IEEE (The Institute of Electrical and Electronics Engineers) 802.11 and/or IEEE802.15 can be named, as an ordinary standard relating to the wireless network.
For example, where IEEE802.11a/g is used, an OFDM (Orthogonal Frequency Division Multiplexing) modulation method which is one of multi-carrier systems is used, as an ordinary standard of wireless LANs. According to the OFDM-modulation method, transmission data is split across a plurality of carriers to which frequencies that are orthogonal to one another are set, and transferred. Therefore, the band of each of the carriers becomes narrow and the frequency-usage efficiency significantly increases, which achieves high endurance for a frequency-selective-phasing interference.
When IEEE802.11a/g standards are used, a modulation method achieving a communication speed of 54 Mbps at the maximum is supported. However, a next-generation wireless-LAN standard capable of achieving a higher bit rate has been demanded. For example, according to IEEE802.11n which is a standard generated by expanding IEEE802.11a/g, a MIMO (Multi-Input Multi-Output) communication is adopted, so as to develop a high-speed wireless-LAN technology achieving an effective throughput higher than 100 MPBS.
The MIMO communication is a technology achieving high-speed radio communications by using spatially multiplexed spatial streams. On the transmission-device side, transmission data is distributed to a plurality of streams and transmitted by using a plurality of antennas, and a reception device performs spatial separation for a spatially-multiplexed signal received through the plurality of antennas by performing signal processing using the channel characteristic, so that a signal can be extracted for every stream without causing any crosstalk (refer to Patent Document 1, for example). According to the MIMO-communication method, the transfer capacity is increased according the number of antennas without increasing the frequency band, whereby an increased communication speed can be achieved.
Further, a PHY layer of IEEE802.11n is provided with a high-throughput (High Throughput: HT) transfer mode (hereinafter referred to as “HT mode”) wherein a transfer method (Modulation and Coding Scheme: MCS) including a modulation method and/or an encoding method is entirely different from that of known IEEE802.11a/g, and the PHY layer is provided with an operation mode wherein data transmission is performed in the same packet format and frequency area as those of known IEEE802.11a/g (hereinafter referred to as “legacy mode”). Further, the HT mode is divided into operation mode that is compatible with a known terminal supporting IEEE802.11a/g (hereinafter referred to as a “legacy terminal”) and that is referred to as “Mixed Mode (MM)”, and operation mode that is not compatible with the legacy terminal at all and that is referred to as “Green Field (GF)” (refer to Non-Patent Document 1, for example).
FIGS. 12 to 14 show packet formats used in the operation modes including the legacy mode, the MM, and the GF, respectively. However, in each of the drawings, a single OFDM symbol is determined to be 4 microseconds.
The format of a packet used in the legacy mode shown in FIG. 12 (hereinafter referred to as a “legacy packet”) is entirely the same as that of IEEE802.11a/g. The header portion of the legacy packet has an L-STF (Legacy Short Training Field) including a known OFDM symbol provided, so as to find a packet, an L-LTF (Legacy Long Training Field) including a known training symbol provided, so as to perform synchronization acquisition and equalization, and an L-SIG (Legacy SIGNAL Field) wherein the transfer rate and/or the data length is written, as a legacy preamble. Subsequently, a payload (Data) is transmitted.
Further, the header portion of a packet shown in FIG. 13 (hereinafter referred to as an “MM packet”) includes a legacy preamble generated in a format which is entirely the same as that used for IEEE802.11a/g, a subsequent preamble generated in a format which is typically used for IEEE802.11n (hereinafter referred to as an “HT format”) (hereinafter referred to as an “HT preamble”), and a data portion. In the MM packet, the portion corresponding to a PHY payload provided in the legacy packet is generated in the HT format, and it may be considered that the HT preamble and the PHY payload are provided in the HT format, reflexively.
The HT preamble includes an HT-SIG, an HT-STF, and an HT-LTF. In the HT-SIG, information necessary to understand the HT format is written, the information showing an MCS and/or the payload-data length used for the PHY payload (PSDU). Further, the HT-STF includes a training symbol provided, so as to increase AGC (automatic-gain control) in a MIMO system. Further, the HT-LTF includes a training symbol provided, so as to perform channel estimation for every input signal subjected to spatial modulation (mapping) on the reception-device side.
Further, when the MIMO communication is performed by using at least two transfer branches, on the reception-device side, a channel matrix needs to be acquired by performing channel estimation for every transmission-and-reception antenna performing spatial separation for a reception signal. Therefore, on the transmission-device side, the HT-LTF is transmitted in a time-division manner from each of transmission antennas. Subsequently, at least one HT-LTF field is added according to the spatial-stream number.
The legacy preamble provided in the MM packet is generated in a format which is entirely the same as that of the preamble of the legacy packet and transferred in a transfer method so that the legacy terminal can decode the legacy preamble. On the other hand, the HT-format portion that comes after the HT preamble is transferred in a transfer method that is not supported by the legacy terminal. The legacy terminal decodes the L-SIG included in the legacy preamble of the MM packet and reads information showing that the MM packet is not addressed to itself, data-length information, and so forth, and sets an NAV (Network Allocation Vector) of an appropriate length, that is, a transmission-wait-time period, so as to avoid a collision. As a result, the MM packet can achieve compatibility with the legacy terminal. However, since the MM packet has the legacy-preamble portion, the format of the MM packet becomes redundant and the MM packet becomes disadvantageous in terms of a throughput.
Further, a packet shown in FIG. 14 (hereinafter referred to as a “GF packet”) includes the HT-format portion only. The preamble of the GF packet includes an L-STF field provided, so as to find a packet, an HT-LTF field provided, so as to perform the channel estimation, an HT-SIG field wherein information necessary to interpret the HT format is written, and a 2nd HT-LTF field. When the MIMO communication is performed, a channel matrix needs to be acquired by performing the channel estimation for every spatial stream. Therefore, the HT-LTFs corresponding to the transmission-antenna number are transmitted in the time-division manner in the 2nd HT-LTF field (same as above). Although the GF packet is not compatible with the legacy terminal at all, the GF packet does not include the legacy preamble. Therefore, the GF packet can achieve a throughput higher than that of the MM packet.
Although support for the Mixed Mode (MM) is absolutely necessary for the currently used IEEE802.11n (EWC), support for the Green Field (GF) is optional. In such cases, it is assumed that two types of MM terminals including an MM terminal supporting the GF and an MM terminal that does not support the GF would be manufactured, as an MM terminal supporting the MM. Hereinafter, processing performed by the MM terminal that does not support the GF, so as to receive the above-described GF packet, will be considered.
First, since the first L-STF of the MM packet is the same as that of the GF packet, the MM terminal can receive the first L-STF without any problem. However, as for subsequent fields, the L-LTF is received, in the case where the MM packet is used. However, the HT-LTF is received in the case where the GF packet is used (refer to FIG. 13 and FIG. 14). The L-LTF is the same as an LTF symbol used for IEEE802.11a. LTF OFDM symbols used at the 20-MHz-operation time and the 40-MHz-operation time are shown by the following expressions (1) and (2), respectively.
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                      L                                    -              26                        ,            26                          =                  {                      1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,            0            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,                          -              1                        ,                          -              1                        ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1                    }                                    (        1        )            
                    [                  Expression          ⁢                                          ⁢          2                ]                                                                      L                                    -              58                        ,            58                          =                  {                      1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,            0            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,                          -              1                        ,                          -              1                        ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,            0            ,            0            ,            0            ,            0            ,            0            ,            0            ,            0            ,            0            ,            0            ,            0            ,            0            ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,            0            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,                          -              1                        ,                          -              1                        ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1                    }                                    (        2        )            
Further, HT-LTF sequences used at the 20-MHz-operation time and the 40-MHz-operation time are shown by the following expressions (3) and (4), respectively.
                    [                  Expression          ⁢                                          ⁢          3                ]                                                                      HTLTF                                    -              28                        ,            28                          =                  [                                                                      1                  ,                  1                  ,                  1                  ,                  1                  ,                                      -                    1                                    ,                  1                  ,                  1                  ,                                      -                    1                                    ,                  1                  ,                                      -                    1                                    ,                  1                  ,                  1                  ,                  1                  ,                  1                  ,                  1                  ,                  1                                                                                                                          -                    1                                    ,                  1                  ,                                      -                    1                                    ,                  1                  ,                  1                  ,                  1                  ,                  1                  ,                  0                  ,                  1                  ,                                      -                    1                                    ,                                      -                    1                                    ,                  1                  ,                  1                  ,                                      -                    1                                    ,                  1                  ,                                                                                                                          -                    1                                    ,                  1                  ,                                      -                    1                                    ,                                      -                    1                                    ,                                      -                    1                                    ,                                      -                    1                                    ,                  1                  ,                  1                  ,                                      -                    1                                    ,                                      -                    1                                    ,                  1                  ,                                      -                    1                                    ,                  1                  ,                                                                                                                          -                    1                                    ,                  1                  ,                  1                  ,                  1                  ,                  1                  ,                                      -                    1                                    ,                                      -                    1                                                                                ]                                    (        3        )            
                    [                  Expression          ⁢                                          ⁢          4                ]                                                                      HTLTF                                    -              58                        ,            58                          =                  {                      1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,            1            ,            1            ,                          1              -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,                          -              1                        ,                          -              1                        ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,                          -              1                        ,            1            ,                          0              ⁢                                                          ⁢              0                        ,                          0              -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1            ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,                          -              1                        ,                          -              1                        ,                          -              1                        ,                          -              1                        ,            1            ,            1            ,                          -              1                        ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,                          -              1                        ,            1            ,            1            ,            1            ,            1                    }                                    (        4        )            
If the MM terminal can recognize the difference between the L-LTF and the HT-LTF, the MM terminal can determine that the GF packet is received at that time. Then, the MM terminal does not perform unnecessary decoding processing in fields that comes after the determination so that the power consumed by reception processing is reduced. However, the L-LTF and/or the HT-LTF does not include mechanisms used for parity check, a CRC, and so forth, and the difference between the sequences is very little. Therefore, the difference between the L-LTF and the HT-LTF cannot be recognized by performing ordinary reception processing.
After that, when the MM packet is used, the L-SIG is received. However, when the GF packet is used, the HT-SIG is received (refer to FIGS. 13 and 14). FIG. 15 shows the format of an L-SIG field. As shown in the drawing, the L-SIG includes the mechanism used for the parity check (even parity for 0 to 16 bits is performed at the 17th bit in the decreasing order). However, since only a single bit is provided, the possibility of erroneously receiving the HT-SIG included in the GF packet, as the L-SIG, is not insignificant. Further, when the HT-SIG is confused with the L-SIG, 5 to 16 of the HT-SIG of the first symbol are read, as Length. Here, when the value read, as Length is longer than the real packet length, the retransmission time delays, which induces inefficient communications. Further, if the MM packet is confused with the GF packet, an unnecessary field is decoded, which wastes operation power of the device.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2002-44051    [Non-Patent Document 1] EWC (Enhanced WIreless Consortium) PHY Specification